PROJECT SUMMARY This project focuses on the underlying basis of two autoimmune fibrotic disorders, IgG4- related disease and systemic sclerosis. We will use multi-color multispectral imaging of disease tissues, cell distance mapping as well as single-cell transcriptomics to interrogate tissues from IgG4-related disease. In particular we wish to identify and quantitate CD4+cytotoxic T cells and activated B cell subsets and better understand whether they directly contribute to cell death in tissues and thus to fibrosis, or if they primarily secrete fibrogenic products. We also want to understand the molecular basis by which CD4+cytotoxic T cells differentiate. We will test two competing but mutually non- exclusive hypotheses - one that CD4+T cells, with the help of activated B cells drive tissue cell death as a prelude to fibrosis, and the other that the cytokines and enzymes secreted by activated T cells and disease-related B cells are causal for fibrosis. We will also examine the interactions between adaptive and innate immune cells in disease and identify antigens that trigger clonally expanded B and T cells in patients. We will also explore the possibility that somatic mutations in hematopoietic stem cells or in CD4+CTL clones contribute to disease pathogenesis. We will also examine whether intestinal microbial gene expression reveals differences between subsets of disease or explains responsiveness to therapy. The possible contribution of microbial metabolites to immune cell differentiation will be examined and microbial antigens that activate host T cells will also be identified.